In heavy lifting or mooring applications, such as marine, oceanographic, offshore oil and gas, seismic, and industrial applications, a standard rope is made from high modulus polyethylene (HMPE) filaments, such as those commercially available under the name of SPECTRA® from Honeywell Performance Fibers of Colonial Heights, Va. and DYNEEMA® from DSM NV of Heerlen, The Netherlands and Toyobo Company Ltd. of Osaka, Japan. These ropes are made into braided ropes or twisted ropes. For example, see U.S. Pat. Nos. 5,901,632 and 5,931,076. Therein is disclosed a braided rope construction in which filaments are twisted to form a twisted yarn, the twisted yarns are braided to form a braided strand, and the braided strands are then braided to form the braided rope.
The type of damage that leads to failure in these ropes is highly dependent on the service conditions, the construction of the rope, but most importantly the type of fibers used to manufacture the rope. When large diameter, high load-capacity ropes are pulled over a drum, pulley, or sheave, as occurs during heavy lifting, e.g. in lowering and raising packages from the seabed, two damage mechanisms are generally observed.
The first damage mechanism is frictional heat generated within the rope. This heat may be caused by the individual elements of the rope abrading one another; as well as, the rope rubbing against the drum, pulley, or sheave. This generated heat can be great enough to cause a catastrophic failure of the rope. This problem is particularly evident when the fiber material loses a substantial amount of strength (or becomes susceptible to creep rupture), when heated above ambient temperature. For example, HMPE fibers exhibit this type of failure; HMPE fibers, however, exhibit the least amount of fiber-to-fiber abrasion.
The second damage mechanism observed during over-sheave cycling of ropes is self-abrasion or fiber-to-fiber abrasion (i.e., rope fibers rubbing against one another). This type of damage is most often observed in ropes made from liquid crystal polymer (LCP) fibers. For example, aramids are known to be a poor material for general rope use because of self-abrasion; aramid fibers, however, are not generally susceptible to creep rupture.
In the studies leading to the instant invention, it was discovered that the primary occurrence of damaging abrasion was at the intersection between the subropes (or strands). Only, a little damage was observed within the subropes. Accordingly, a way to reduce the abrasion between the subropes was investigated.
In the prior art, jacketing the subropes is a known method for reducing abrasion between the subropes. Jacketing refers to the placement of a sleeve material (e.g., woven or braided fabric) over the subrope, so that the jacket is sacrificed to save the subrope. These jackets, however, add to the overall diameter, weight and cost of the rope without any appreciable increase in the rope's strength. The larger size is obviously undesirable because it would require larger drums, pulleys, or sheaves to handle the jacketed rope. In addition, rope jackets make visual inspection of the rope core fibers problematic because the jacket hides the core fibers. Therefore, while this solution was viable, it was considered unsatisfactory.
Accordingly, there is a need for a new rope solution, one without a jacket on the subropes that could be used in heavy lifting or mooring applications and have a reduced risk of failure. This rope solution would have to be resistant to creep rupture (unlike a rope made entirely from HMPE) and also resistant to self-abrasion (unlike a rope made entirely from LCP).
Small diameter rope (i.e., diameters less than or equal to 1.5 inches or 34 mm) made of blends of HMPE filaments and liquid crystal polymer filaments selected from the group of lyotropic and thermotropic polymer filaments are known. New England Ropes of Fall River, Mass. offers a high performance double braided rope (STA-SET T-900), consisting of blended SPECTRA® filaments and TECHNORA® filaments core within a braided polyester jacket, having a diameters up to 1.5 inches (34 mm). Sampson Rope Technologies of Ferndale, Wash. offers two yacht racing ropes: VALIDATOR SK, a double braid construction having a blended, urethane coated core of VECTRAN® filaments and DYNEEMA® filaments within a braided polyester jacket in diameters up to 0.75 inches (17 mm); and LIGHTNING ROPE, a twelve-strand single braid construction having a urethane coating and made from blended DYNEEMA® filaments and VECTRAN® filaments in diameters up to 0.625 inches (16 mm). Gottifredi Maffioli S.p.A. of Novara, Italy offers high performance halyards (DZ) of a double braid construction having a composite braid made of ZYLON® filaments and DYNEEMA® filaments within a jacket in diameters up to 22 mm.
In these small diameter ropes, the reason for blending HMPE and LCP fibers is to reduce creep elongation, and not to improve high-temperature fatigue life. For example, the yachting ropes cited above are used in halyards where dimensional stability (low to no creep) is critical for consistent sail positioning. HMPE ropes are more commonly used in small sailing ropes, however for the halyard application the creep of 100% HMPE fiber is considered prohibitive. Blending HMPE with LCP fibers greatly reduces the creep elongation in the product. Reduction of creep elongation in the core of these core/jacket products also prevents the core from bunching after elongating relative to the jacket. Blending the low-creep LCP fibers with the low-cost HMPE fibers also reduces the manufacturing cost of these products.
Moreover, all of those small diameter blended rope designs would have severe limitations if scaled to larger sizes. All are constructed with braided or extruded outer jackets. Although adequate in sizes ≦1.5 inches diameter, jacketed designs are less able to shed the tremendous amounts of heat that can be generated in larger ropes subjected to rapid bend cycling as over sheaves. Furthermore, jacketed designs limit the ability of the owner to assess damage done from heating or internal abrasion.
Finally, several of the prior art designs utilize parallel fiber, yarn, or strand as the core strength member. Designs that use parallel yarns or strands in the core are also subject to tensile overloads in the outer strands and compression kinking in the inner strands when subjected to bending over small radii sheaves and drums. This problem becomes more pronounced as rope size increases.